1. Technical Field of the Invention
The present invention relates to switch mode power supplies and more specifically to those having only one inductive element and several regulated outputs. The invention is advantageously applicable to, but not limited to, portable systems powered by battery, in particular, but not exclusively, wireless remote communication system terminals, for example, cellular mobile telephones.
2. Description of Related Art
Integrated systems for controlling the power aspects of portable applications (battery operated) increasingly require different output supply voltages to be regulated. The traditional solution consists in creating a regulated supply from the power supply using a linear regulator. This regulator uses an external capacitor to fulfill its function. Further, as many linear regulators are used as there are outputs to be regulated. The main drawback of this type of regulator is its low efficiency.
To address this problem of low efficiency, switch mode power supplies, also known as “DC—DC converters,” are used. Efficiency levels are then close to 85 to 90%, while under the same conditions, a linear regulator would have an efficiency of only 50%. However, the main drawback of a switch mode power supply is the need for both an external capacitor and an external inductive element.
To regulate several outputs, it is conceivable to use several switch mode power supplies. However, this then assumes using as many external inductive elements, the main drawback of such an approach being the cost of such an inductive element and the large surface area taken up by it on the printed circuit board.
It has also been considered to use switch mode power supplies capable of regulating several separate outputs using only one inductive element. The main solutions that exist in this field then rely on a time-division multiplexing of the power delivered to the various outputs. In other words, the various outputs to be regulated are fed respectively during successive conduction cycles.
The prior art solutions exhibit a number of drawbacks.
One of these concerns the implementation of a “discontinuous conduction” type regulation on the various outputs. Also, this type of conduction gives rise to high peak currents in the inductive element, thus resulting in a significant drop in efficiency (65 to 75% instead of 85 to 90%). Furthermore, to withstand the very high peak currents, the saturation current of the inductive element must be higher. Consequently, an inductive element having a larger volume must be used, which is a major constraint for portable applications. Furthermore, this type of conduction produces significant ripple on the regulated output voltage supplies. Another drawback concerns the use of complex digital functions taking up considerable surface area. These functions are for controlling the power delivered at the various outputs according to given algorithms. Another drawback concerns the fact that the conduction is, for most of the existing solutions, a pulse frequency modulation type conduction. This principle leads to significant and uncontrollable spectral pollution. Also, although in some applications, such as applications for supplying power to microcomputers or liquid crystal display screens, such a pollution is not critical, this pollution is unacceptable in other portable applications, such as cellular mobile telephones, and this means that switch mode power supplies using frequency modulation cannot be used.